1. Field of the Invention
The invention is related to an improved process for the preparation of largely isocyanurate-free uretediones from (cyclo)aliphatic diisocyanates. Such uretediones can be processed into light-fast, single and two component polyurethane paints. The presence of isocyanurates is undesired in a number of applications since, as well-known, they are trifunctional and have a tendency to form crosslinkages. In practice an admixture of more than 0.5% is regarded as undesirable.
2. Discussion of the Background
In principle it is well-known that uretediones can be prepared in the presence of specific catalysts through dimerization of isocyanates. Previously for this purpose antimony pentafluoride, trialkylphosphines, amino-substituted phosphines, imidazoles, guanidines, and pyridines have been proposed.
The drawback in the use of antimony pentafluoride (cf. DE-OS No. 34 20 114) is that this corrosive and expensive compound must be destroyed prior to distillation with a five-fold quantity of zinc powder and the antimony and zinc fluoride precipitate must be removed by means of filtration.
A process is known from FR-PS No. 15 32 054 in which tertiary phosphines or boron trifluoride are added as dimerization catalysts. However, they catalyze not only the dimerization but also to a significant degree also the trimerization of isocyanates. In addition to this, due to its high corrosiveness, boron trifluoride can be added only when specific protective measures are taken.
1,2-Dimethylimidazole is an excellent dimerization catalyst for aromatic isocyanates (cf. Synthesis 1975, p. 463 ff.). However, in the case of isocyanates that do not have any aromatic NCO groups, this catalyst is clearly less selective. For example, when benzyl isocyanate is added, a mixture of 24% isocyanurate and only 76% uretedione is obtained.
In practice amino-substituted phosphines have prevailed as dimerization catalysts.
According to the process of DE-OS No. 30 30 513, the uretedione of the isophorone diisocyanate is prepared in the presence of an organic phosphorus-nitrogen-catalyst by means of dimerization of monomers and then distilled in a thin-layer evaporator. The uretediones remain in the distillation residue while the unconverted monomers with the majority of the added catalyst are collected as distillate, which is recycled into the process. Tris-(N,N-dimethylamino)-phosphine (PTD) is designated as the preferred catalyst. Uretediones, which were produced in the presence of PTD, contain typically 1% isocyanurate, 1% monomers, and 0.01-0.1% catalyst.
In the DE-OS No. 34 47 635 it is proposed that for the dimerization of organic isocyanates the same type of catalysts be used with concurrent use of active hydrogen containing organic compounds such as alcohols, phenols, (cyclo)aliphatic amines, amides, urethanes, and ureas. PTD and tris-(N,N-diethylamino)-phosphine are particularly preferred catalysts. If di- or higher functional isocyanates are added, the reaction usually stops after attaining a degree of dimerization ranging from 10 to 50% due to the addition of a catalyst poison such as chloroacetic acid (cf. page 16, first paragraph). Isolating the uretedione presents a problem. Under the conditions of thin-layer evaporation there is the risk of a catalyzed dissociation of the uretedione that is present back to the original isocyanates (cf, page 16, middle). If the catalyst with the excess isocyanate can be removed by means of distillation, deactivation of the catalyst is superfluous. However, then uncontrollable secondary reactions can occur during and after work-up (cf, page 20).
It is known that PTD has a tendency to react in the presence of atmospheric oxygen to form hexamethyl triamidophosphoric acid, which as is well-known, is suspected to be a carcinogen (cf. Br.J.Cancer 38, 418-427 (1978)). Therefore, the use of PTD should be avoided. In addition to this, the aforementioned processes have the drawback that the catalysts enter into secondary reactions and thus are partially consumed. Therefore, in these processes, the lost catalyst must be regularly replaced. In the latter processes, a significant proportion of the catalyst thus employed remains in the uretedione after the deactivation. Therefore, a drawback of both processes is the high cost of the catalyst.
Other dimerization catalysts are also known from the literature. For example, in the JP-AS No. 71/37 503 the dimerization of 2,4-toluylene diisocyanate with cyclic amidines such as 1,8-diazabicyclo[5.4.0]undec-7-en is described. Experiments conducted by the Applicant show that (cyclo)aliphatic diisocyanates cannot be dimerized with this catalyst (see reference example A). Apparently the well-known, low reactivity of these diisocyanates is inadequate to facilitate a reaction.
The object of DE-PS No. 10 81 895 (corresponding to U.S. Pat. No. 3,144,452) is a process for the preparation of N,N-diaryluretediones and triarylisocyanurate acid esters through di- or trimerization of aromatic isocyanates. Pyridines containing a substituent in the 3- or 4-position and of a specified basicity are used as catalysts. Among other things, 4-aminopyridines that are substituted by means of alkyl groups are mentioned. According to this process, it is apparently possible to obtain uretediones, isocyanurates or their mixtures, depending on the quantity of the catalyst, the reaction temperature, and type of solvent that is used. Thus, for example, it is recommended that for the production of uretediones the catalyst be added in a quantity ranging from 0.005 to 15%, with respect to the weight of the isocyanate added, the mixture be reacted at the lowest temperature possible, and an inert organic solvent be used in which the uretedione dissolves poorly.
The fact that the quantity of catalyst recommended for the preparation of isocyanurates overlaps in broad ranges the aforementioned data and the other two reaction parameters are also not clearly delineable, leads one skilled in the art to doubt the possibility of controlling a selective reaction. Reference experiments conducted by the Applicant show in fact that the two oligomers are always formed. For example, in the follow-up of Example 2, 2% by weight of isocyanurate was obtained. If the reaction mixture is heated briefly to 145.degree. C., even 10% by weight of isocyanurate is obtained, whereas at the same time the uretedione is partially split into the monomer.
It is evident from a later application of the patent holder that 4,4'-diphenylmethane-diisocyanate is trimerized in the presence of 4-dimethylaminopyridine at room temperature and is converted to higher oligomeric products (cf. DE-AS No. 16 94 485). From this, too, it can be inferred that apparently the pyridine derivatives always catalyst both reactions--the dimerization and the trimerization. Therefore, pyridine derivatives do not seem to be suitable for the preparation of uretediones, which should be almost free of isocyanurates. In particular, this applies to aliphatic isocyanates, since in contrast to the aromatic isocyanates, they form either no uretediones or only when special reaction conditions are maintained are they in a position to form the dimeric addition products (cf. J.Org.Chem. 36, 3056 (1971)).
Therefore, whereas numerous processes for the dimerization of aromatic diisocyantes are known, there is practically only one possibility for dimerizing (cyclo)aliphatic diosocyanates; and it is based on the use of undesired aminophosphines (see DE-OS No. 34 37 635)).
The process described in JP-OS No. 84/98180 for the oligomerization of (cyclo)aliphatic diisocyanates is not suitable for the targeted preparation of uretediones, since it is known that the mixtures obtained comprising uretediones and isocyanurates are separable only with great difficulty. Uretediones have the tendency, on heating to reseparate back into their original components. In particular, this can be expected when catalyst residues are still present.